Biomechanics of Bone

Characteristics of the Skeletal System

  • Purpose of the Skeletal System:

    • Protects internal organs, providing a protective casing for vital structures such as the heart, lungs, and brain.

    • Provides rigid kinematic links and muscle attachment sites that facilitate efficient movement by acting as levers for muscle action.

    • Facilitates muscle action and body movement, essential for locomotion and maintaining posture.

  • Bone Properties:

    • Unique structure with mechanical properties tailored to support its roles, consisting of a combination of density and elasticity.

    • Among the hardest biological materials, with strength exceeding that of most soft tissues, rivaling materials used in engineering applications.

    • Highly vascular, capable of self-repair, and adaptively alters its properties based on mechanical demands and loading conditions over time.

    • Bone density can change significantly with disuse (leading to osteoporosis) or increased usage (resulting in hypertrophy).

    • The adaptation of bone shape and structure during healing or certain surgical operations highlights its dynamic nature, allowing for recovery and optimization based on functional demands.

Bone Composition and Structure

  • Components of Normal Human Bone:

    • Mineral/Inorganic Portion:

      • Primarily composed of calcium and phosphate.

      • Forms hydroxyapatite Ca10(PO4)6(OH)2, a crystalline structure that provides rigidity and strength.

      • Accounts for approximately 60-70% of the dry weight of bone, critical for load-bearing capabilities.

    • Water:

      • Constitutes about 5-8% of total composition, contributing to bone's viscoelastic properties.

    • Organic Matrix:

      • Includes collagen and non-collagenous proteins that provide flexibility and tensile strength to the bone, allowing it to withstand various mechanical loads over time.

  • Bone Composition Breakdown:

    • Inorganic Components: 65-70% (dry weight), contributing to compressive strength.

    • Water: 25-30%, important for metabolic activities and maintaining bone health.

    • Organic Components:

      • Contains collagen, which is viscoelastic, ductile, and possesses brittle properties under different stress conditions.

Types and Structure of Bones

  • Types of Human Bones:

    • Long Bones:

      • Characterized by a shaft (diaphysis) that is significantly longer than its width, facilitating movement and bearing weight (e.g., femur, humerus).

    • Short Bones:

      • Roughly cube-shaped with a thin layer of compact bone surrounding a spongy interior, providing stability with little movement (e.g., carpals and tarsals).

    • Flat Bones:

      • Thin and curved, consisting of two parallel layers of compact bone with spongy bone in between, serving protective roles and offering surfaces for muscle attachment (e.g., skull, sternum).

    • Irregular Bones:

      • Have complex, irregular shapes that do not fit into other categories, crucial for protecting nervous tissue and providing anchor points for muscles (e.g., vertebrae, pelvic bones).

    • Sesamoid Bones:

      • Embedded in tendons, providing protection and mechanical advantages (e.g., patella, which helps in knee extension).

Structural Elements of Bone

  • Collagen Organization:

    • Arranged in a parallel structure with defined gap zones that influence the degree of mineralization, critical for maintaining bone strength and flexibility.

  • Bone Structure Components:

    • Osteoblasts:

      • Forming cells responsible for bone formation and mineralization, play a vital role in the growth and healing of bones.

    • Osteoclasts:

      • Destroying cells that resorb bone tissue, essential for bone remodeling and calcium homeostasis.

    • Osteocytes:

      • Inactive osteoblasts embedded in bone matrix that maintain bone tissue and communicate mechanical stress signals.

  • Extracellular Matrix:

    • Composed of collagen (organic phase) and hydroxyapatite (mineral phase), providing mechanical strength and elasticity necessary for load-bearing functionality.

Mechanical Properties of Bone

  • Bone Characteristics:

    • Exhibits high compressive strength with relatively lower tensile strength, making it capable of withstanding vertical loads but requiring careful design to address tensile forces.

    • Its lightweight yet tough design stems from the effective combination of varying collagen types and mineral content, optimizing strength-to-weight ratios.

  • Density and Stress Properties:

    • Cancellous bone is more elastic and has lower density than cortical bone, allowing for energy absorption during impact.

    • Bone adapts to mechanical loads through a remodeling process (Wolff's law), which facilitates continuous change in density and structure based on usage patterns.

Fractures and Healing

  • Types of Bone Fractures:

    • Complete:

      • Bone snaps entirely into separate pieces, requiring careful realignment for healing.

    • Incomplete:

      • Bone cracks but remains partially intact, often seen in children (e.g., greenstick fractures).

    • Compound:

      • Bone breaks through the skin, increasing the risk of infections and requiring surgical intervention.

    • Simple:

      • Bone breaks but does not penetrate through skin, generally allowing for less complication in healing.

    • Types of Injuries:

      • Greenstick, Transverse, Oblique, Comminuted, Impacted fractures—each type presents its unique challenges in treatment and recovery.

  • Bone Repair Process:

    1. Hematoma Formation:

      • Blood infiltrates the fracture site, creating a blood clot that serves as a foundation for healing.

    2. Soft Callus Development:

      • Formation of fibrocartilage, bridging the fracture gap to stabilize the bone temporarily.

    3. Hard Callus Formation:

      • Osteoblasts produce new bone tissue that forms a hard callus, gradually replacing the soft callus.

    4. Bone Remodeling:

      • Osteoclasts remove excess bone and callus, reshaping the bone back to its original form and restoring structural integrity.

Osteoporosis and Joint Disorders

  • Osteoporosis:

    • Characterized by low bone mass and microarchitectural deterioration, significantly increasing fracture risk, particularly in elderly populations.

    • Symptoms include chronic aches, loss of height over time, and frailty fractures, mainly affecting the hip, spine, and wrist.

    • Risk factors include age, genetic predisposition, lifestyle factors (e.g., inadequate nutrition, sedentary behavior, smoking, and alcohol consumption).

  • Arthritis Types:

    • Rheumatoid Arthritis:

      • Chronic autoimmune disorder resulting in joint inflammation, pain, and potential joint destruction.

      • Symptoms include pain, joint stiffness, disability, and potential systemic effects on other organs.

    • Osteoarthritis:

      • A degenerative joint disease resulting from the wear and tear of cartilage, characterized by loss of cushioning and joint function.

Biomechanical Properties of Bone

  • Composition:

    • Bone functions as a two-phase composite material, combining mineral/inorganic phases with collagen/organic phases for optimal performance under various loading conditions.

  • Strength Characteristics:

    • Strength properties can vary significantly based on species, age, loading direction, and rate of load application, necessitating careful consideration during biomechanical analysis.

  • Stress-Strain Behavior:

    • Bone exhibits elastic properties under normal loading but can become brittle under rapid loading conditions, emphasizing the importance of controlled loading in injury prevention.

  • Remodeling:

    • Bone adapts to mechanical stress through a dynamic remodeling process involving resorption (removal) and deposition (formation) of bone tissue, maintaining functional integrity over time.